ghc/docs/users_guide/vs_haskell.sgml

   1 <Sect1 id="vs-Haskell-defn">
   2 <Title>Haskell&nbsp;98 vs.&nbsp;Glasgow Haskell: language non-compliance
   3 </Title>
   4
   5 <Para>
   6 <IndexTerm><Primary>GHC vs the Haskell 98 language</Primary></IndexTerm>
   7 <IndexTerm><Primary>Haskell 98 language vs GHC</Primary></IndexTerm>
   8 </Para>
   9
  10 <Para>
  11 This section lists Glasgow Haskell infelicities in its implementation
  12 of Haskell&nbsp;98.  See also the &ldquo;when things go wrong&rdquo; section
  13 (<XRef LinkEnd="wrong">)
  14 for information about crashes, space leaks, and other undesirable
  15 phenomena.
  16 </Para>
  17
  18 <Para>
  19 The limitations here are listed in Haskell-Report order (roughly).
  20 </Para>
  21
  22 <Sect2 id="infelicities-exprs-pats">
  23 <Title>Expressions and patterns
  24 </Title>
  25
  26 <Para>
  27 <VariableList>
  28
  29 <VarListEntry>
  30 <Term>Very long <Literal>String</Literal> constants:</Term>
  31 <ListItem>
  32 <Para>
  33 May not go through.  If you add a &ldquo;string gap&rdquo; every
  34 few thousand characters, then the strings can be as long
  35 as you like.
  36 </Para>
  37
  38 <Para>
  39 Bear in mind that string gaps and the <Option>-cpp</Option><IndexTerm><Primary>-cpp option</Primary></IndexTerm>
  40 option don't mix very well (see <XRef LinkEnd="c-pre-processor">).
  41 </Para>
  42 </ListItem>
  43 </VarListEntry>
  44 <VarListEntry>
  45 <Term>Very long literal lists:</Term>
  46 <ListItem>
  47 <Para>
  48 These may tickle a &ldquo;yacc stack overflow&rdquo; error in the parser.
  49 (It depends on the Yacc used to build your parser.)
  50 </Para>
  51 </ListItem>
  52 </VarListEntry>
  53 <VarListEntry>
  54 <Term>Single quotes in module names:</Term>
  55 <ListItem>
  56 <Para>
  57 It might work, but it's just begging for trouble.
  58 </Para>
  59 </ListItem>
  60 </VarListEntry>
  61 </VariableList>
  62 </Para>
  63
  64 </Sect2>
  65
  66 <Sect2 id="infelicities-decls">
  67 <Title>Declarations and bindings
  68 </Title>
  69
  70 <Para>
  71 <VariableList>
  72
  73 <VarListEntry>
  74 <Term>Derived instances for records:</Term>
  75 <ListItem>
  76 <Para>
  77 Hmmm.
  78 </Para>
  79 </ListItem>
  80 </VarListEntry>
  81 </VariableList>
  82 </Para>
  83
  84 </Sect2>
  85
  86 <Sect2 id="infelicities-Modules">
  87 <Title>Module system and interface files
  88 </Title>
  89
  90 <Para>
  91 <VariableList>
  92
  93 <VarListEntry>
  94 <Term> Namespace pollution </Term>
  95 <ListItem>
  96 <Para>
  97 Several modules internal to GHC are visible in the standard namespace.
  98 All of these modules begin with <Literal>Prel</Literal>, so the rule is: don't use any
  99 modules beginning with <Literal>Prel</Literal> in your program, or you will be
 100 comprehensively screwed.
 101 </Para>
 102 </ListItem>
 103 </VarListEntry>
 104 </VariableList>
 105 </Para>
 106
 107 </Sect2>
 108
 109 <Sect2 id="infelicities-numbers">
 110 <Title>Numbers, basic types, and built-in classes
 111 </Title>
 112
 113 <Para>
 114 <VariableList>
 115
 116 <VarListEntry>
 117 <Term>Unchecked arithmetic:</Term>
 118 <ListItem>
 119 <Para>
 120 Arguably <Emphasis>not</Emphasis> an infelicity, but&hellip; Bear in mind that
 121 operations on <Literal>Int</Literal>, <Literal>Float</Literal>, and <Literal>Double</Literal> numbers are
 122 <Emphasis>unchecked</Emphasis> for overflow, underflow, and other sad occurrences.
 123 (note, however that some architectures trap floating-point overflow
 124 and loss-of-precision and report a floating-point
 125 exception)<IndexTerm><Primary>floating-point exceptions</Primary></IndexTerm>.
 126 </Para>
 127
 128 <Para>
 129 Use <Literal>Integer</Literal>, <Literal>Rational</Literal>, etc., numeric types if this stuff
 130 keeps you awake at night.
 131 </Para>
 132 </ListItem>
 133 </VarListEntry>
 134 <VarListEntry>
 135 <Term>Multiply-defined array elements&mdash;not checked:</Term>
 136 <ListItem>
 137 <Para>
 138 This code fragment <Emphasis>should</Emphasis> elicit a fatal error, but it does not:
 139
 140 <ProgramListing>
 141 main = print (array (1,1) [ 1:=2, 1:=3 ])
 142 </ProgramListing>
 143
 144 </Para>
 145 </ListItem>
 146 </VarListEntry>
 147 </VariableList>
 148 </Para>
 149
 150 </Sect2>
 151
 152 <Sect2 id="infelicities-Prelude">
 153 <Title>In Prelude support
 154 </Title>
 155
 156 <Para>
 157 <VariableList>
 158
 159 <VarListEntry>
 160 <Term>Arbitrary-sized tuples:</Term>
 161 <ListItem>
 162 <Para>
 163 Plain old tuples of arbitrary size <Emphasis>do</Emphasis> work.
 164 </Para>
 165
 166 <Para>
 167 HOWEVER: standard instances for tuples (<Literal>Eq</Literal>, <Literal>Ord</Literal>, <Literal>Bounded</Literal>, <Literal>Ix</Literal>
 168 <Literal>Read</Literal>, and <Literal>Show</Literal>) are available <Emphasis>only</Emphasis> up to 5-tuples.
 169 </Para>
 170
 171 <Para>
 172 These limitations are easily subvertible, so please ask if you get
 173 stuck on them.
 174 </Para>
 175 </ListItem>
 176 </VarListEntry>
 177 <VarListEntry>
 178 <Term>Unicode character set:</Term>
 179 <ListItem>
 180 <Para>
 181 Haskell 98 embraces the Unicode character set, but GHC doesn't
 182 handle it. Yet.
 183 </Para>
 184 </ListItem>
 185 </VarListEntry>
 186 </VariableList>
 187 </Para>
 188
 189 </Sect2>
 190
 191 </Sect1>